Natural magnetism is acquired by some rocks, especially igneous rocks that are rich in iron, as they solidify. As the rocks cool, iron-bearing minerals are 'frozen' into position. The orientation of iron-bearing magnetic minerals record the location of the Earth's magnetic poles and the latitude of the rock at the time of cooling.

A very coarse-grained igneous rock, commonly with a granitic composition. Usually forms from molten rock rich in water or other volatiles that facilitate the growth of large crystals. Forms sills and dikes.

The ability of a rock or other material to allow water to flow through its interconnected spaces. Permeable bedrock makes a good aquifer, a rock layer that yields water to wells. See porosity. (3 MB porosity animation available)

A very fine-grained, foliatedmetamorphic rock, generally derived from shale or fine-grained sandstone. Phyllites are usually black or dark gray; the foliation is commonly crinkled or wavy. Differs from less recrystallized slate by its sheen, which is produced by barely visible flakes of muscovite (mica).

A member of the feldspar mineral family. Plagioclase feldspars are silicates that contain considerable sodium and calcium. Feldspar crystals are stubby prisms, generally white to gray and a glassy luster.
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Playas are shallow, short-lived lakes that form where water drains into basins with no outlet to the sea and quickly evaporates. Playas are common features in arid (desert) regions and are among the flattest landforms in the world.
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The earliest Epoch of the Quaternary Period, beginning about 1.6 million years ago and ending 10,000 years ago. Commonly known as the 'Ice Age', a time with episodes of widespread continental glaciation.
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The percentage of open spaces (pores) in rock or soil. When these spaces are interconnected, water, air, or other fluids can migrate from space to space. Interconnected spaces make the soil or bedrock permeable.

Mineral precipitate. A mineral deposited from a water solution in pores or other openings in rocks. Chemical reaction with the surrounding rock, changes in pressure or temperature, or just drying up (evaporation) can cause a mineral to precipitate out of solution. Quartz veins are common products of mineral precipitation.

An igneous rock texture produced from consolidation of fragmented volcanic material ejected during a violent eruption. Also used to describe ash, bombs and other material forcefully ejected during a volcanic eruption (=tephra)
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A volcanic eruption that produces a large volume of solid volcanic fragments (pyroclastics) rather than fluid lava. This type of eruption is typical of volcanoes with high silica, viscous, gas-rich magma.

Family of silicate minerals containing iron, magnesium, and calcium in varying amounts. Differ from amphibole family by lack of water in the crystals. The most common variety, augite, contains aluminum as well. Generally forms very dark green to black stubby prisms.